Our site is served from www.brfcs.com and also just plain brfcs.com ... this causes an issue for a few users with the sheer size of some of the  cookies that advertisers set (it's the consent ones mainly, they're massive!

Can I set `COOKIE_DOMAIN` in my constant.php to force the domain to always be just `brfcs.com` ? The www subdomain should pick up it's cookies from that too shouldn't it?

Yes, but this only controls cookies IPS4 itself sets (so anything prefixed with ips4_, or your prefix if you've changed it previously). Cookies from external sources will follow their own rules.

If you do this, make sure you use two dots as that's what allows the cookies to be read by subdomains:

define( 'COOKIE_DOMAIN', '.brfcs.com' );

 

Actually, that's a good point, it's the consent and advertising cookies causing ther issues, not IPS ones.

set up a redirect from www to the none www domain job done